NEW YORK (CNN) -- New York police detectives are reviewing surveillance video from Columbia University in hopes of identifying the person or persons who hung a noose on a professor's office door earlier in the week.

The noose was found Tuesday, hanging from a door at Columbia's Teachers College.

"NYPD detectives began to download videotape images from security cameras on the campus ... after serving a subpoena on campus officials who declined to release the material without one," Deputy Commissioner Paul J. Browne said in a statement Thursday.

Investigators said they have about 56 hours of video they want to review from Tuesday's incident, which is being investigated as a hate crime.

According to police, no arrests have been made and no suspects have been identified. The noose was described as being made of twine rope three-quarters of an inch in diameter.

The apparent target was Madonna Constantine, 44, an African-American professor of psychology and education at the Teachers College.

She co-authored the book "Addressing Racism: Facilitating Cultural Competence in Mental Health and Educational Settings."

Don't Miss

Constantine issued a statement on Tuesday saying, "I am upset that the Teachers College community has been exposed to such an unbelievably vile incident, and I would like us to stay strong in the face of such a blatant act of racism.

Joe Levine, executive director for external affairs at the Teachers College, said the building, which is open around the clock, is accessible only to those with a Teacher's College ID card or other credentials. He said it appeared the noose was placed on the door before 9 a.m.

Police said they received a 911 call about the noose about 9:45 a.m. Tuesday. They were told it was on the fourth floor of the Teachers College.

Levine said that although the entrance to the building is covered by security cameras, the hallway where the noose was found is not.

According to school officials, there are more than 5,000 graduate students and 165 faculty members in Teachers College.

In a separate development, the Hate Crime Task Force opened an investigation into the appearance of another noose on scaffolding outside a post office under renovation in lower Manhattan. U.S. Postal Service employees noticed the noose from a second story office and reported it to police Thursday afternoon.

Columbia University is located about eight miles to the north of the post office on the west side of uptown Manhattan. E-mail to a friend